JP2878294B2 - Lithium battery - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium battery in which discharge composition is improved by optimizing the composition of a positive electrode mixture and a pressurized density.

<Conventional technology> In a lithium battery of an inside-out type, a coin type, or the like, graphite is mixed as a conductive agent with manganese dioxide as an active material, and polytetrafluoroethylene is mixed as a binder. A positive electrode mixture obtained by press-molding the compounded mixture at a high pressure is used.

Conventionally, as a positive electrode mixture molded under pressure as described above,
A material having a molding density about twice the bulk density of the compounded mixture is used.

In other words, conventionally, from the viewpoint that the richer the electrolytic solution, the better the discharge performance of the battery, the mixture molding is performed loosely within a range that does not hinder the battery assembly process, and the liquid absorption in the positive electrode mixture is performed. The electrolyte is enhanced to absorb a large amount of electrolyte.

<Problems to be Solved by the Invention> However, when such a positive electrode mixture having a low molding density is used, the mixture swells significantly during discharge, and absorbs the electrolyte more than necessary. For this reason, there is a problem that the electrolytic solution is depleted as the discharge proceeds, the internal resistance of the battery increases, and the discharge performance sharply decreases.

<Means for Solving the Problems> The present inventor has studied to solve the above problems,
Surprisingly, when the molding density of the positive electrode mixture is equal to or higher than a predetermined value larger than the conventional value, it is possible to effectively suppress such a decrease in the discharge performance due to the swelling of the mixture, thereby improving the electric performance of the battery. It was learned to get.

The present invention is based on such knowledge, and a compounding agent obtained by mixing manganese dioxide as an active material, graphite as a conductive agent, and polytetrafluoroethylene as a binder to a predetermined bulk density. In a lithium battery using a positive electrode mixture obtained by granulating and pressing this granulated mixture to a predetermined molding density, the ratio of graphite in the positive electrode mixture is 1 to 20% by weight, and the ratio of polytetrafluoroethylene is Is 1 to 5% by weight, and the molding density is at least 2.5 times the bulk density.

The reason why the amounts of graphite and polytetrafluoroethylene in the positive electrode mixture are specified as described above is as follows.

That is, if the amounts of these components are too large, the absolute capacity of the battery is reduced and the discharge performance is reduced. If the amount of graphite is less than this range, the capacity of the battery itself increases, but the absolute amount of the conductive agent decreases, and therefore, the conductivity of the battery decreases, the internal resistance of the battery increases, and the discharge performance decreases. On the other hand, if the blending amount of polytetrafluoroethylene is less than this, the mixture cannot be molded properly, and the strength of the mixture after molding cannot be obtained.

<Action> By defining the molding density of the positive electrode mixture as described above,
Swelling of the positive electrode mixture during discharge progress is effectively suppressed, and depletion of the electrolyte is prevented. As a result, an increase in the internal resistance of the battery due to discharging is suppressed, and the discharging performance of the battery is improved.

Further, by increasing the amount of graphite, the conductivity of the positive electrode mixture is improved, and the utilization rate of the positive electrode active material is improved.

Embodiment An embodiment of the present invention will be described below with reference to the accompanying drawings.

Manganese dioxide (MnO ₂ ), graphite (Gr) and polytetrafluoroethylene (PTFE)
As shown in FIG. 1 (A), a mixture 1 prepared by mixing at the compounding ratio (% by weight) shown in the table was compressed with rollers 2 and 3 respectively.
To form a sheet mixture 4.

Then, the sheet-shaped mixture 4 is pulverized and sieved to form a granulated mixture A having a particle size of 25 to 80 mesh.
F was obtained. In addition, the bulk density (g / cm) of these granulated mixes A to F
³ ) is also shown in Table 1.

Next, as shown in FIGS. 1 (B) to 1 (C), each of these granulated mixes 5 is incorporated into the concave molding surface 6a of the mold 6, and granulated by the following method. The loading amount of the agent was adjusted.

Then, the granulated mixture 5 is removed from the upper surface of the molding surface 6a by, for example, dropping a punch having a shape corresponding to the mold 6 into the first form.
As shown in Fig. (D), the positive electrode mixture 7 was formed into a hollow cylindrical positive electrode mixture 7 having an outer diameter of 16 mm, an inner diameter of 12 mm, and a height of 15 mm.

At this time, in FIG. 1 (D), the punch was applied to the surface (compressed surface) indicated by reference symbol P, and the above-mentioned molding was performed so that the molding pressure on this compressed surface was 5 ton / cm ² .

Then, the positive electrode mixture is molded in this manner, and various positive electrode mixtures having a molding density / bulk density ratio changed in a range of about 2 to 3 are produced. Five inside-out type lithium batteries each having the structure shown in FIG. 2 were manufactured. In this figure,
8 is a battery can, 9 is a separator, 10 is a lithium negative electrode, 11 is a lead terminal, 12 is a sealing plate, 13 is a terminal plate, and 14 is a sealing gasket.

Table 2 shows the molding density (g / cm ³ ) and the ratio of molding density / bulk density in the positive electrode mixtures obtained from these granulated mixtures A to F.

Each of these batteries A to F was prepared in a quantity of 5 pieces, and at a temperature of 20 ° C., the discharge resistance was 2.7 kΩ and the voltage was constant throughout.
They were continuously discharged to 2.0 V and their discharge duration was examined. The results are as shown in Table 3 (average value of 5 samples).

On the other hand, various inside-out type lithium batteries were produced in the same manner except that the molding pressure on the compression surface was set at ² ton / cm ^2, and these batteries were continuously discharged under the same conditions, and the discharge duration time was examined.

The above experimental results were plotted by taking the discharge duration on the vertical axis and the molding density / bulk density ratio on the horizontal axis for each of the molding pressures, and the graph shown in FIG. 3 was obtained.

From the figure, it is understood that good discharge performance can be obtained when the ratio of molding density / bulk density is about 2.5 or more. From this result, it can be seen that when the molding pressure on the molding surface is 5 ton / cm ² or more, good discharge performance can be obtained.

By the way, the mixture ratio at each measurement point in the graph of FIG. 3 is different. In other words, the smaller the ratio of molding density / bulk density, the smaller the amount of the conductive agent (in this case, graphite), while the larger the ratio, the more the conductive agent.

That is, the bulk density of the mixture obtained by granulation decreases as the amount of the conductive agent increases. On the other hand, when molding, in order to obtain a positive electrode mixture of a certain size, the amount of the granulated mixture charged into the molding die of the molding machine, that is, the filling amount is adjusted. (Which can be adjusted by the molding machine).

Those having a large bulk density, that is, those having a small amount of graphite, have a small amount of filling (volume) and a small compression ratio (volume of filling / volume of granulated mixture). Conversely, when the amount of graphite is large, the compression ratio increases. As a result, the ratio of molding density / bulk density increases because the decrease in bulk density is greater than the decrease in molding density due to an increase in the amount of graphite.

At this time, the weight of the positive electrode mixture is different in each point, and the theoretical capacity of the positive electrode decreases as the amount of graphite increases, but the conductivity of the mixture increases accordingly, thereby improving the battery performance.

Although the above is an example of a cylindrical lithium battery, the same applies to, for example, a case where the present invention is applied to a coin-type lithium battery, and a disk-shaped positive electrode mixture used for the battery is similarly subjected to pressure molding. It is clear that excellent results can be obtained.

<Effect of the Invention> According to the present invention configured as described above, the swelling of the positive electrode mixture during the progress of discharge is effectively suppressed, and therefore, the increase in internal resistance due to discharge is suppressed, and the discharge performance of the battery is reduced. The effect that improvement can be achieved is produced.

[Brief description of the drawings]

1 (A) to 1 (D) are explanatory diagrams of a manufacturing procedure of a positive electrode mixture in an example, FIG. 2 is a sectional view of a battery in the example, and FIG. 3 shows discharge characteristics of each battery in the example. FIG. 1 ... combination mixture, 5 ... granulation mixture, 7 ... positive electrode mixture, 8
…… Battery can, 10… Lithium negative electrode.

Continuation of the front page (72) Inventor Hideaki Yagi 5-36-11 Shimbashi, Minato-ku, Tokyo Inside Fuji Electric Chemical Co., Ltd. (56) References JP-A-63-175349 (JP, A) JP-A-57- 61259 (JP, A) JP-A-57-34652 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) H01M 4/06-4/08 H01M 6/16 Z

Claims (1)

(57) [Claims] 1. A mixture obtained by mixing manganese dioxide as an active material, graphite as a conductive agent, and polytetrafluoroethylene as a binder is granulated to a predetermined bulk density. In a lithium battery using a positive electrode mixture obtained by press-molding an agent to a predetermined molding density, the ratio of graphite in the positive electrode mixture is 1 to 20% by weight, and the ratio of polytetrafluoroethylene is 1 to 5% by weight.
Wherein the molding density is at least 2.5 times the bulk density.